Colored contact lenses are used to change the appearance of color of the eyes of the wearer and there are currently two dominant methods for coloring such lenses. In one method successive layers of coloring are applied to a mold surface in which the mold is filled with a liquid (composed of monomers and/or macromers) that harden into a lens upon curing. In the second method, a plurality of colored inks is successively applied to the outer surface of the formed contact lens and cured in place.
Since eye contact is an important element of human interaction, it is important that a colored contact lens impart an aesthetically pleasing colored print design to the eye of a wearer. To create such a complex pattern, a plurality of colors, each with a pattern of very fine dots, are applied to the surface of each lens or to the surface of a mold used to form a lens. To create the pattern for each color, a cliché is formed with the pattern of dots etched in the surface of the cliché. Colored ink is then applied into the etchings of the cliché, and when a print pad is subsequently pressed against the surface of the cliché, the ink in the etchings of the cliché is transferred to the surface of the print pad. After the pattern has been applied to the surface of the print pad, the print pad is pressed against the smooth surface of a contact lens or a contact lens mold to transfer the ink from the print pad to the smooth surface thereof.
A various pad transfer printing pads can be used to print ink for contact lenses (for example, U.S. Pat. Nos. 5,452,658 and 6,979,419. However, various problems may exist when using these known printing pads for the product of contact lenses. The print pads that are currently in use do not last more than 20,000 impressions before the print pads are no longer usable and must be replaced. The time used to stop the printing machine and replace the pad transfer printing pads is lost time and must be charged to the overall cost of manufacturing printed contact lenses. In addition, the printing pad softness is adjusted by adding silicone oil, which is known to affect print quality. However, high levels of silicone oil can transfer onto the contact lens, resulting in non-wetting spots during wear. There is also a problem that some print pads have shown toxicity, possibly due to tin-based catalyst components or impurities in its formulations. This could affect the biocompatibility of the resulting contact lens. Another problem with the print pads previously used was the poor adhesion. The old print pads would transfer the ink to the lens, but the ink would not always adhere permanently to the lens.
It would be desirable and advantageous to create a pad transfer printing pad with an optimum hardness to be used for contact lenses that would accept the ink from a source and release the ink freely to the lens (more affinity than the ink source but less affinity than the lenses), that would last for more 20,000 transfers, and still retain all the positive aspects of the prior art. The savings of such a pad transfer printing pad would be twofold: an initial savings for a reduced number of print pads to make the same number of contact lens impressions, and secondly, the time saved on a work shift that previously was used to change the print pads frequently.
It would be desirable if the improved print pad provided no toxicity to the contact lens and improve the biocompatibility of the contact lens.
It would also be desirable if the print pads had a reduced silicone oil residue release to enhance the print quality.